1. Field of the Invention
The present invention relates to an automatic gain controller.
2. Description of the Related Art
In a conventional automatic gain controller having a filter in a control loop, an output signal of the conventional automatic gain controller is often interrupted due to a delay of response characteristics thereof caused by a signal delay from the filter. In order to solve the above problem, a method has been proposed, wherein if a signal level has a sudden variation, a control signal generator having a rapid time constant is utilized to create a control signal to cope with a delay of the control signal caused by the filter, and at the same time, control information is copied between the control signal generators in such a manner that an output signal of an automatic gain controller has no variation in level (referred to in Japanese Patent No. 3,240,458).
In addition, in the conventional automatic gain controller having the filter in the control loop, if a level control of an automatic gain control is carried out according to in-band signals of the filter, the automatic gain control may be carried out to match levels of the in-band signals of the filter. Accordingly, if out-of-band signals of the filter are greater than in-band signals of the filter, saturation caused by the out-of-band signals of the filter may occur in an amplifier installed at a front end of the filter. For this reason, a method has been proposed in which amplifiers are cascade-connected through a filter and an automatic gain control is separately carried out with respect to each amplifier installed at front and rear portions of the filter (referred to in Japanese Patent No. 3,086,060).
Although above-described Japanese Patent Publication No. 3,240,458 discloses a technique capable of solving a delay of response characteristics caused by a signal delay from a filter and properly generating an output signal of an automatic gain controller, when a level control of an automatic gain control is carried out according to in-band signals of the filter, saturation may occur in an amplifier installed at a front end of the filter caused by out-of-band signals of the filter, if out-of-band signals of the filter are greater than in-band signals of the filter.
In addition, although above-described Japanese Patent Publication No. 3,086,060 can adjust a signal level at a front end of a filter by tracking variations of levels of out-of-band signals of the filter, if an automatic gain control for the amplifier installed at the front end of the filter is carried out with a high-speed response according to variation of out-of-band signals of the filter, unnecessary variation may occur in the in-band object signals of the filter even though the object signals are not subject to variation.
That is, if the automatic gain control for the amplifier installed at the front end of the filter is carried out with the high-speed response according to the variations of the signals, the in-band object signals of the filter may be subject to unnecessary variation, so the automatic gain control for the amplifier installed at the rear end of the filter may be carried out according to the variations of the object signals with a high-speed response characteristic. However, if the automatic gain control with respect to the object signals is carried out with the high-speed response, signals may be modulated due to the automatic gain control and signal distortion may occur caused by an amplitude compression action of the automatic gain control.
Therefore, according to above-described Japanese Patent publication No. 3,086,060, the automatic gain control for the amplifier installed at the front end of the filter must be carried out with a low-speed response according to the variations of out-of-band signals of the filter. In such a case, noise is generated by the out-of-band signals of the filter due to low response characteristics of the automatic gain control. If the automatic gain control is carried out at a high-speed with respect to both amplifiers installed at front and rear ends of the filter, distortion of the object signals may occur when the automatic gain control is carried out with respect to the amplifier installed at the rear end of the filter, so communication quality will be lowered.